The flash-photolysis resonance fluorescence technique was used to study the reaction kinetics of hydroxyl radicals with ten aromatic and six olefinic hydrocarbons at 298 K and several diluent gas pressures. The aromatic compounds that were studied include benzene, toluene, ethylbenzene, n-propylbenzene, isopropylbenzene, hexafluorobenzene, n-propyl pentafluorobenzene, and o-, m-, and p-xylenes; and the olefins include ethylene, acetylene, propylene, 1-butene, cis-2-butene, and tetramethylethylene. Based on our extensive data on OH-substituted aromatic hydrocarbon reactions, it has been inferred that addition of hydroxyl radicals to the aromatic ring is the dominant reaction in these systems. In the case of OH-olefin reactions, addition of OH to the double bond seems to be a prominent path for the heavier unsaturates. From these rate constant data, the lifetimes of all these hydrocarbons in the lower troposphere has been calculated. Attempts were made to understand the mechanisms involved in the reactions of OH with substituted aromatic hydrocarbons, using the technique of laser flash photolysis, time-of-flight mass spectrometry.